1. Field of the Invention
This invention relates to a liquid crystal display, and more particularly to a ferroelectric liquid crystal display having improved liquid crystal alignment films.
2. Description of the Related Art
A liquid crystal display (LCD) controls the light characteristics of a screen to display a desired image. Liquid crystals used in LCDs are in a neutral state between a liquid and a solid. That neutral state has both fluidity and elasticity.
While there are many types of liquid crystals, one type of great interest is the smectic C liquid crystal. During a thermodynamic phase transition, a smectic C liquid crystal rotates along an outer line of a virtual cone. Such a smectic C phase liquid crystal can undergo a spontaneous polarization. Such a liquid crystal is usually referred to as a “ferroelectric liquid crystal” (FLC). The FLC has been actively studied because of its fast response time. Furthermore, FLC LCDs can have wide viewing angles without the complications of special electrode structures or compensating films.
There are many different FLC modes, including a deformed helix FLC, a surface stabilized FLC, an anti-FLC, a V-mode FLC and a half V-mode FLC. Hereinafter, the V-mode FLC mode and the half V-mode FLC mode will be described in more detail.
FIG. 1 illustrates a liquid crystal cell having a V-mode FLC. As shown, that liquid crystal cell includes an upper substrate 1 having a common electrode 3 and an alignment film 5. That liquid crystal cell also includes a lower substrate 11 having a TFT array 9, which includes pixel electrodes, and an alignment film 7. A V-mode liquid crystal 13 is interposed between the upper and lower substrates 1 and 11. The alignment films 5 and 7 are aligned in a horizontal direction, usually by rubbing with a special cloth roller. The V-mode liquid crystal 13 forms multiple smectic layers that have molecular structures arranged with desired slopes with respect to a plane perpendicular to the smectic layers. In other words, the liquid crystal molecules have desired inclination angles with respect to the alignment directions of the alignment films. Furthermore, adjacent smectic layers have opposite polarities.
Light transmission through the V-mode FLC liquid crystal cell varies according to an applied voltage across that cell, reference FIG. 2. The liquid crystal 13 within the V-mode FLC liquid crystal cell responds to both positive and negative voltages. Since light transmissivity rapidly changes in accord with applied positive and negative voltages, the light transmissivity verses voltage curve has a V-shape as shown in FIG. 2. Thus, light transmissivity increases regardless of polarity.
FIG. 3 illustrates a liquid crystal cell having a half V-mode FLC. As shown, a half V-mode FLC liquid crystal 15 is interposed between an upper substrate 1 and a lower substrate 11. The half V-mode FLC liquid crystal 15 forms multiple smectic layers in which the liquid crystal molecules align at a desired inclination angle with respect to the alignment direction of the alignment films 5 and 7. However, as shown in FIG. 3, the liquid crystal molecules in adjacent smectic layers have the same polarity (unlike V-mode FLC liquid crystal molecules). Such a half V-mode FLC liquid crystal can be formed by applying a positive (or a negative) electric field to a hot liquid crystal while slowly lowering that liquid crystal's temperature into a smectic phase.
A half V-mode FLC mode liquid crystal 15 formed in this manner responds to only one polarity of applied voltage. Thus, as shown in FIG. 4, the light transmissivity verse voltage curve of a liquid crystal cell having the half V-mode FLC has a ‘half V’ shape. Still referring to FIG. 4, as shown, the light transmissivity verse voltage curve does react, slightly, to negative applied voltages, but dramatically to positive applied voltages.
The thermodynamic phase transition of a half V-mode FLC liquid crystal 15 is as follows:Isotropic→nematic (N*) phase→smectic C*(Sm C*) phase→crystalSuch a thermodynamic phase transition expresses the phase of the liquid crystal in accordance with temperature, which becomes less as phase changes move to the right.
An isotropic phase liquid crystal 15 interposed into a liquid crystal cell aligns in parallel with the rubbing direction of the alignment layers when the liquid crystal temperature is slowly lowered to the nematic phase. If a sufficiently strong electric field is applied across the liquid crystal cell while the liquid crystal temperature is slowly lowered more, the liquid crystal 15 is phase-changed into a smectic phase in which the direction of spontaneous polarization of the liquid crystal molecules arranges according to the electric field in the cell. Consequently, when the liquid crystal 15 within the liquid crystal cell is subject to parallel alignment treated alignment layers, the liquid crystal molecules arrange in a spontaneous polarization direction that is consistent with the electric field at the phase transition, and in one of two possible molecular arrangements. As a result, the liquid crystal 15 has a uniform alignment state.
FIG. 5 and FIG. 6 help illustrates this. First, as shown in FIG. 5, if a negative electric field E(−) is applied during alignment of the liquid crystal 15, then the spontaneous polarization direction of the liquid crystal 15 is along the electric field. In such an aligned liquid crystal cell, as shown in FIG. 6, the liquid crystal arrangement is changed by an applied positive electric field E(+), but not by an applied negative electric field E(−).
To utilize the response characteristics of the liquid crystal 15, perpendicular polarizers are arranged on the upper and lower portions of the liquid crystal cell. The transmission axis of one of the polarizers is along the direction of the initial liquid crystal alignment. Assuming a liquid crystal cell having the transmission curve of FIG. 4, an applied negative electric field E(−) does not change the liquid crystal arrangement and the perpendicular polarizer blocks light. A positive electric field E(+) rotates the liquid crystal alignment such that light transmission increases.
FIG. 7 shows an electric field applied across a half V-mode FLC liquid crystal cell. As shown, the half V-mode FLC liquid crystal cell includes an upper substrate 1 with a common electrode 3 and an alignment film 5, a lower substrate 11 with a TFT array 9 and an alignment film 7, and a liquid crystal 15 interposed between the upper and lower substrates 1 and 11. The alignment films 5 and 7 are beneficially comprised of the same material and are subject to the same alignment treatment. An internal electric field, E-intra, which is contrary to the externally applied electric field, E-ext, depends on the alignment film material and on the polarization of the liquid crystal 15. This internal electric field E-intra is an induced polarization field.
As described above, the half V-mode FLC liquid crystal cell uses both temperature and an electric field to obtain the initial liquid crystal alignment. However, liquid crystal cells made in this manner have a problem in that the initial liquid crystal alignment can be disturbed by external impacts, which almost inevitably occurs due to grinding of a shorting bar. Furthermore, simple heating of a conventional half V-mode FLC liquid crystal cell can disturb the liquid crystal alignment. To re-establish liquid crystal alignment, both temperature and electric field treatments are required, which is difficult to do without the shorting bar.
Therefore, a new ferroelectric liquid crystal display having a liquid crystal alignment that can be thermally re-established would be beneficial.